1. Field of the Invention
This invention relates to focusing screens for observing an image formed by an objective lens, and more particularly to focusing screens for single lens reflex cameras.
2. Description of the Prior Art
A focusing screen for single lens reflex cameras using a liquid crystal operating in a dynamic scattering mode is shown, for example, in Japanese patent publication No. SHO 48-37379 (Published Nov. 10, 1974). The scattering degree of such a focusing screen can be made arbitrarily adjustable depending on the F value of the photographic lens, as shown, for example, in Japanese laid-open patent application No. SHO 50-115523 (published Sept. 10, 1975).
The focusing screen disclosed in the above-mentioned Japanese laid-open patent application No. SHO 50-115523 operates its entire surface area when a voltage is applied to the liquid crystal layer. The amount of scattering by the focusing screen is adjusted by varying the magnitude of the voltage applied to the liquid crystal layer. Therefore, it is very difficult to maintain the scattering degree of the focusing screen at a desired value. FIG. 1 represents the amount of light transmitted through a prior art liquid crystal having a dynamic scattering effect versus the applied voltage, where the incident light is white and normal to the liquid crystal layer surface and the transmittance of this incident light taken as 100% when no voltage is applied. As is obvious from this graph, a liquid crystal having a dynamic scattering effect has a rapid change in transmission as the applied voltage gradually increases. Therefore, adjusting the scattering degree of a conventional focusing screen to, for example, 70% transmittance has been difficult. Additionally, the voltage which this rapid change in transmittance occurs depends largely on the temperature. Accordingly, maintenance of the applied voltage at a constant level does not always result in a desired diffusion characteristic.